Abstract
Background: When performing total hip arthroplasty without computer navigation, surgeons align the acetabular component with landmarks such as the plane of the operating table and the presumed position of the pelvis. In contrast, first-generation computer navigation systems rely on the pelvic anterior plane, defined by the anterior superior iliac spines and the pubic tubercle. We sought to study the effect of patient positioning on the tilt of the pelvis as measured in the pelvic anterior plane and its effect on cup alignment angle values.
Methods: In forty patients, the supine pelvic anterior plane tilt angle was measured with use of computed tomographic scans made before and after total hip arthroplasty (Group A). In thirty other patients undergoing total hip arthroplasty, preoperative supine pelvic anterior plane tilt angle was measured with a computed tomographic scan and the preoperative standing pelvic anterior plane tilt angle was measured on a lateral radiograph (Group B). From these data, we used hip navigation planning software to develop a nomogram providing tilt-adjusted cup angles that would align the cup in a target range of 40° ± 10° of abduction and 15° ± 10° of anteversion. A third group of ninety-eight patients (Group C) then underwent total hip arthroplasty with computer navigation with use of our nomogram to provide tilt-adjusted values for cup alignment. Postoperative computed tomography scans were made to evaluate cup alignment, and the patients were followed for at least one year.
Results: In Group A, the mean preoperative supine pelvic tilt angle (and standard deviation) was -8.9° ± 6.8° (forward rotation of the pelvis) and the mean postoperative angle was -10.9° ± 7.6° (p < 0.05). In Group B, the mean preoperative supine pelvic tilt angle was -10.4° ± 7.4° and the mean preoperative standing pelvic tilt angle was -5.0° ± 9.4° (p < 0.001). In the group of ninety-eight patients who underwent navigated total hip arthroplasty (Group C), there were no dislocations at one year of follow-up. Seventy-two patients underwent postoperative computed tomography scans; 99% of cup anteversion values and 97% of cup abduction values were in the target range.
Conclusions: For navigation systems that rely on the pelvic anterior plane, cup alignment values can be converted to familiar target values with our nomogram with good accuracy and reproducibility. The next generation of navigation systems should be able to measure the pelvic tilt for each individual patient and automatically adjust alignment values.
Level of Evidence: Diagnostic Level II. See Instructions to Authors for a complete description of levels of evidence.
Malpositioning of one or both components and soft-tissue imbalance are important factors influencing the prevalence of impingement, dislocation, polyethylene wear, and aseptic loosening of hip prostheses1-4. With a prevalence of 2% to 3% following primary total hip replacement and as high as 10% after revision total hip replacement, dislocation is second in frequency only to aseptic loosening as a cause for revision surgery and remains a substantial clinical problem5-7.
Almost thirty years ago, Lewinnek et al.3 made recommendations for cup alignment based on measurements of cup position on anteroposterior radiographs. They attempted to standardize the position of the pelvis on the table by maintaining the pelvic anterior plane (a plane through both anterior superior iliac spines and the pubic tubercle8) parallel to the plane of the operating table. The deviation of the pelvic anterior plane from the plane of the table determined the pelvic tilt angle. Later, Murray9 described a set of nomograms demonstrating the relationship between anatomical, radiographic, and operative measurements of cup abduction and anteversion, but Murray assumed a constant pelvic tilt angle for all patients.
The true spatial orientation of the acetabulum requires an assessment of the pelvic tilt angle8,10. However, during total hip arthroplasty, the true position of the pelvis cannot be determined by the surgeon with accuracy11-13. The optimum acetabular component position is still a controversial issue, but most orthopaedic surgeons continue to use alignment goals similar to those proposed by Lewinnek et al.3 and endeavor to align the cup at 40° ± 10° of abduction and 15° ± 10° of anteversion in relation to osseous landmarks and/or the plane of the operating room table, without knowledge of the patient's actual pelvic tilt angle.
Within the past ten years, computer-assisted surgery has been introduced for hip replacement. The aim is to achieve a more precise alignment of both components, but especially the acetabular component. This new technology allows implantation of the cup with a precision of about ±5° in clinical practice14-17. Navigation systems orient themselves in relation to the pelvic anterior plane.
Currently, little is known about the influence of pelvic tilt on anteversion and abduction of the cup. The purpose of the present study was to systematically analyze the effect of the pelvic tilt angle on acetabular component placement. First, we evaluated the supine pelvic tilt angle for patients undergoing total hip arthroplasty. We also evaluated the differences in tilt angle between the standing and supine positions. We then built a nomogram allowing us to convert the abduction and anteversion angles measured during cup navigation into values more easily understood by the surgeon by referencing them to the plane of the operating room table. Finally, we implanted cups with computer navigation with use of the converted angles from our nomogram in order to optimize placement of the acetabular component according to the patient's pelvic tilt angle. We then followed our patients for at least one year.
We received approval from the ethics committee of our University and obtained informed consent from participating patients. The examinations were performed on different groups of patients to minimize overall radiation exposure to any individual patient.
Evaluation of Pelvic Tilt Angle in the Supine Position
Forty consecutive patients (Group A) who were candidates for unilateral total hip arthroplasty underwent computed tomographic scanning preoperatively and six weeks postoperatively to determine the pelvic tilt angle in the supine position. Candidates for inclusion in the study included patients who were undergoing primary total hip arthroplasty for the treatment of developmental dysplasia or primary coxarthrosis who had a flexion contracture of the hip joint of <10°. Thirty patients (thirty hips; 75%) had a diagnosis of developmental dysplasia, and ten patients (ten hips; 25%) had a diagnosis of primary coxarthrosis. The computed tomography scans were performed on a SOMATOM Balance (Siemens, Erlangen, Germany). The standard acquisition protocol, with the gantry at 0° and a slice thickness of 1.0 to 1.5 mm, has been validated on cadaver specimens14; abduction and anteversion angle measurements have been shown to be accurate to within 0.7°. The planning module of the image-based hip navigation system Navitrack (ORTHOsoft, Montreal, Quebec, Canada) was used to create a three-dimensional model of the pelvis from the scans and to measure the pelvic tilt angles preoperatively and six weeks postoperatively.
In order to measure inclination and abduction angles relative to various pelvic tilt angles, reference coordinate systems must be used. As surgeons are familiar with making measurements relative to the plane of the operating room table, we referenced the conventional coordinate system to be parallel to the plane of the computed tomography scan table (Fig. 1). When the conventional coordinate system is used, cup abduction and anteversion are referenced to the computed tomography scan table in the same way in which a surgeon orients the cup in relation to a standard operating room table during conventional total hip arthroplasty. However, navigation systems use a coordinate system that is based on the pelvic anterior plane (Fig. 1), which we have renamed the anatomical coordinate system.
To measure the pelvic tilt angle relative to the conventional coordinate system, three-dimensional models of the pelvis were aligned to obtain a perfect lateral position (Fig. 2, left). The conventional coordinate system is parallel to the table, and the pelvic tilt angle is defined as the angle between the conventional coordinate system and the anatomical coordinate system as measured with our mediCAD/EndoMap (Hectec, Niederviehbach, Germany and Siemens, Erlangen, Germany) software package. An anterior tilt of the anatomical coordinate system was defined as "forward pelvic rotation" and was assigned a negative value. A posterior tilt of the anatomical coordinate system in relation to the conventional coordinate system was defined as "backward pelvic rotation" and was assigned a positive value.
Evaluation of the Change in Pelvic Tilt Angle Between Supine and Standing Positions
Changes in the pelvic tilt angle between the supine and standing positions were measured in thirty patients (thirty hips) (Group B) who met the same inclusion criteria as those in Group A. Hip surgery was performed for the treatment of developmental dysplasia in seventeen patients (57%) and primary coxarthrosis in thirteen patients (43%). The patients in Group B also were evaluated with use of a preoperative plain lateral radiograph that was made with the patient in the standing position (Fig. 2, right). The standing radiograph was made with the x-ray tube perpendicular to the floor and centered over the greater trochanter. With the same mediCAD software package, we measured the angle between the anatomical coordinate system and a line vertical to the floor. Anterior rotation of the anatomical coordinate system relative to this vertical axis was also defined as "forward pelvic rotation" and was given a negative value. We compared the supine tilt angle measured on computed tomography (Fig. 2, left) and the standing tilt angle measured on radiographs (Fig. 2, right).
Building the Nomogram to Convert Conventional Coordinate System Angles to Anatomical Coordinate System Angles
The computed tomographic scan of a female patient was used to produce a three-dimensional pelvis model with use of the same protocol14. From that model, we could then simulate the effect of different pelvic tilt angles on cup alignment in the anatomical coordinate system. With use of the planning module of the computed tomography-based Navitrack system (ORTHOsoft), the pelvis model was positioned with a neutral pelvic angle. Thus, the conventional coordinate system was parallel to the anatomical coordinate system, simulating a pelvic tilt angle of 0° (Fig. 3, left). Then we virtually implanted a 50-mm hemispherical cup into the acetabulum of the model (Fig. 3, left) with 40° of abduction and 15° of anteversion. We then imposed increasing forward rotation to the pelvis model in 5° increments but kept the cup in its orientation in relation to the conventional coordinate system. The software directly displayed corresponding values for abduction and anteversion as measured relative to the anatomical coordinate system (Fig. 3, right). We plotted these values for pelvic rotation from -30° (forward rotation) to +20° (backward rotation) on a nomogram. In order to provide curves for multiple cup positions, we repeated our procedure with cup positions between 50° and 30° of abduction and between 30° and 10° of anteversion.
The effect of pelvic tilt angle on cup alignment angles relative to the anatomical coordinate system is shown in Figure 4 for cups implanted at 40° of abduction and 15° of anteversion relative to the operating room table (the conventional coordinate system). The lines of safe abduction ±5° and anteversion ±5° determine the optimal cup position. The interval between -5° and +5° in relation to this line describes a "safe zone" of cup position for the anatomical coordinate system. Thus, for a pelvic tilt of -15° (forward rotation), when a surgeon places the cup impactor in the usual 40° of abduction and 15° of anteversion relative to the conventional coordinate system, the computer navigation systems will display 44° of abduction and 26° of anteversion as measured relative to the anterior pelvic plane (the anatomical coordinate system) (Fig. 4). When the navigation system is used, the target values for abduction and anteversion relative to the anatomical coordinate system can be determined once the pelvic tilt angle has been evaluated at the time of surgery. They should lie in the safe zone as shown in Figure 4. Other target values are presented in the Appendix, where we have plotted various cup positions varying from 50° abduction/30° anteversion to 30° abduction/10° anteversion.
Navigation with Corrected Angle Values
In another group of ninety-eight patients (Group C), we performed a series of ninety-eight total hip replacements with use of the Navitrack (ORTHOsoft) and VectorVision (BrainLAB, Feldkirchen, Germany) systems to check the validity of the nomogram. In all instances, the operation was performed through an anterolateral transgluteal approach with the patient in the supine position. The individual tilt angles were measured with the navigation system at the time of surgery. In order to implant the cup at 40° of abduction and 15° of anteversion when referenced to the usual operating room table plane, we used the nomogram to provide us with the necessary abduction and anteversion angles as calculated in the anatomical coordinate system for the patient's actual pelvic tilt angle. Finally, we measured the postoperative cup angles in seventy-two patients (73%) who agreed to undergo a postoperative computed tomographic scan of the pelvis with use of the same computed tomography protocol14. With the same methodology, a three-dimensional pelvic model was reconstructed for each of those seventy-two patients and the cup alignment was measured in the anatomical coordinate system with use of the planning module of the computed tomography-based navigation system. All ninety-eight patients were followed for at least one year in order to determine stability of the replacement and to document any dislocation event.
Statistical Methods
All data were analyzed statistically with use of SPSS software (SPSS, Chicago, Illinois). This analysis included a test for distribution with use of the Shapiro-Wilk test and was followed by a Wilcoxon signed-rank test or t test. The Pearson correlation coefficient was used to evaluate associations between two continuous quantitative variables. The level of significance was set at p < 0.05.
Evaluation of Pelvic Tilt Angle in a Supine Position
In Group A, the mean supine pelvic tilt angle was -8.9° ± 6.8° (range, -24° to 10°) preoperatively and -10.9° ± 7.6° (range, -27° to 7°) postoperatively (p < 0.05) (Table I). The correlation coefficient between the preoperative and postoperative pelvic tilt angles in the supine position was 0.82 (p < 0.001). As seen in Figure 5, both preoperative and postoperative pelvic tilt angles followed a skewed distribution in forward rotation and the pelvic angles were not centered around 0°. A multivariate analysis was performed to determine the influence of gender and diagnosis. The postoperative inclination of the pelvis was significantly higher in women as compared with men (p < 0.05). Forward rotation was also significantly higher (p < 0.05) in patients with hip dysplasia than in patients with primary osteoarthritis.
The individual change of the pelvic position was determined by the difference between preoperative and postoperative pelvic tilt angles. The mean difference between preoperative and postoperative pelvic tilt was 1.9° ± 4.4° (range, -5° to 15°) (Table I). Diagnosis and gender did not significantly influence this value. Thirteen patients (33%) had a change in backward rotation of the pelvis (mean change, 2.6° ± 1.5°), and twenty-three patients (58%) had a change in forward rotation of the pelvis (mean change, 4.8° ± 3.4°). In four hips (10%), no change of position was measurable. Despite high intersubject variability, the preoperative-to-postoperative change in the supine pelvic tilt angle was <10° for most patients, with thirty-nine of forty patients falling into the category of a "stable" or "stiff" pelvic position.
Evaluation of the Change in Pelvic Tilt Angle Between Supine and Standing Positions
The mean preoperative pelvic tilt angle in the supine position was significantly different from the mean preoperative tilt angle in the standing position (-10.4° ± 7.4° compared with -5.0° ± 9.4°; p < 0.001) (Table II). The Pearson correlation coefficient between pelvic tilt angles in the supine and standing positions was 0.88 (p < 0.001). Plots of the pelvic tilt angles measured in this group showed a wider spread of pelvic tilt in the standing position (Fig. 5, bottom right) than in the supine position (Fig. 5, bottom left) as well as a shift of the curve to the right, corresponding with backward rotation.
Patients with hip dysplasia showed a significantly higher pelvic forward rotation angle in both the supine position (-13.8°) and the standing position (-9.4°) as compared with patients with primary arthritis (p < 0.05). No significant gender-specific differences were found.
The change in pelvic tilt from the supine to the standing position in each patient in Group B was defined as pelvic mobility. The mean mobility was 5.4° ± 4.6° (Table II). Diagnosis and gender did not influence pelvic mobility. Additional analysis showed backward pelvic rotation in twenty-six of thirty patients during transition from the supine position to the standing position and showed forward rotation of the pelvis in only four patients. We again found a relatively stiff pelvis, with a low change of pelvic tilt angle of <10°, in twenty-five of the thirty patients.
Navigation with Corrected Angle Values
Ninety-eight patients (ninety-eight hips), including seventy women and twenty-eight men with a mean age of 56.8 years (range, 31.2 to 81.1 years) underwent computer-assisted total hip arthroplasty with use of the Navitrack system (ORTHOsoft) (sixty-six hips) or the VectorVision system (BrainLAB) (thirty-two hips). All cups (including sixty-six Allofit press-fit cups and thirty-two Duraloc press-fit cups) were implanted after evaluation of the pelvic tilt with the patient in the supine position and with the abduction and anteversion angles given by the nomogram (Fig. 4).
Figure 6 shows the cup position for all seventy-two patients (seventy-two hips) who agreed to undergo a postoperative computed tomography scan. In two patients (3%), the abduction angle fell outside the optimal range of between -10° and +10° in relation to the line of safe abduction as defined by our nomogram. In one patient (1%), the anteversion angle fell outside the optimal range of between -10° and +10° in relation to the target anteversion as defined by our nomogram. All other patients showed abduction and anteversion values that corresponded to the anatomical coordinate system target value ±10°.
At one year of follow-up, none of the ninety-eight patients in Group C had sustained a dislocation.
Several methods of analysis of the spatial orientation of hip prostheses are currently in use18-22. Most of them use the conventional coordinate system as a reference for cup alignment. Recently, image-based and imageless computer-navigation technology has been developed with use of the anatomical coordinate system as a new reference for cup alignment. Despite disadvantages such as cost, increased operating time, and the risk of loosening of reference markers, the advantages of two-dimensional or three-dimensional planning such as the high precision of component placement and restoration of limb length are notable13,16,17,23.
The so-called safe zone for cup position as defined by Lewinnek et al.3 uses the conventional coordinate system as a reference, but those authors did not analyze the effects of pelvic tilt on cup alignment. Thus, application of these guidelines to a navigation system is inaccurate if the pelvis is not in neutral rotation. Unfortunately, most hip navigation systems until now have ignored the problem of pelvic tilt.
Our results clearly demonstrate that the average pelvic tilt angle when the subject is supine is not neutral; it is negative, indicating forward rotation. We also found a high intersubject variability in the pelvic tilt angle of between 27° of forward rotation preoperatively and 10° of backward rotation postoperatively. Multivariate analysis showed that women had significantly greater forward pelvic tilt rotation than men, by an average of 4.2°. Similarly, multivariate analysis showed that patients with a dysplastic hip had significantly greater forward pelvic tilt angle rotation than those with coxarthrosis, by an average of 4.7°.
When we compared the preoperative and postoperative pelvic tilt angles in the supine position in Group A, we found a relatively constant position of the pelvis, with individual differences of <10°, a finding consistent with those of previous studies24,25. This small postoperative change in the pelvic tilt angle therefore should not increase the risk of hip dislocation.
We also found that there was a significant change in pelvic tilt angle during the transition from the supine position to the standing position, with a mean backward rotation of 5.4°. The range of pelvic movement between the supine and standing positions did not exceed 10° in 83% of the hips. These results confirm that the supine pelvic tilt angle is a reasonable approximation of the standing pelvic tilt angle, which is the position of the pelvis for many activities of daily living.
When angles are measured relative to the anatomical coordinate system, the nomogram presented in Figure 4 shows the dramatic influence that pelvic tilt can have on acetabular component orientation. Anteversion changes approximately 4° for every 5° of change in the pelvic tilt angle. Abduction is less affected, changing approximately 1.5° for every 5° of change in the pelvic tilt angle. For example, for a forward pelvic tilt of -15°, anteversion of 26° and abduction of 44° will be required in the anatomical coordinate system in order to have a final anteversion of 15° and abduction of 40° in the usual conventional coordinate system. The difference is very important clinically, particularly for the anteversion value. Thus, if the surgeon were to navigate the cup into a position of 15° instead of 26° of anteversion, the cup position would be in almost neutral anteversion in the conventional coordinate system, perhaps increasing the risk of a posterior dislocation.
Computer navigation can help to increase the reproducibility of surgery, provided that the data are interpreted correctly. When using a navigation system that refers only to the anatomical coordinate system, the surgeon must adjust the anteversion and abduction cup angles to the patient's pelvic tilt angle to implant the cup in the so-called safe zone (abduction, 40° ± 10°; anteversion, 15° ± 10°) according to the conventional coordinate system. In contrast, if the surgeon relies on constant targets for every patient and ignores pelvic tilt, navigation systems that refer only to the anatomical coordinate system will give inaccurate readings26.
When a conventional surgical technique is used, the exact component orientation of the cup can be difficult to achieve in patients with coxarthrosis secondary to developmental dysplasia. In our consecutive series of ninety-eight patients, sixty-one had a diagnosis of developmental dysplasia of the hip. As shown in Figure 6, most cups fell within the so-called safe zone, with a markedly limited scatter of angles. The clinical results illustrate the importance of considering the value of the pelvic tilt angle when implanting a cup relative to the anatomical coordinate system used in computer navigation systems.
For all surgical approaches in which the patient is in the supine position, our technique is straightforward. For posterolateral approaches with the patient in the lateral decubitus position, surgery begins with the patient in the supine position in order to digitize the anatomical coordinate system and to register the pelvic tilt angle. The pelvic tracker is then protected by a sterile plastic drape, and the patient is repositioned in the lateral position.
Unusual anatomical features such as acetabular bone defects or increased femoral anteversion occasionally can force the surgeon to place the cup in a slightly different orientation than the usual safe position. The nomograms presented in the Appendix allow conversion of different cup alignments to tilt-adjusted abductions and anteversions for navigation in the anatomical coordinate system. For example, for a pelvic tilt angle of -20°, in order to achieve a socket position of 50° of abduction and 20° of anteversion as measured in the conventional coordinate system, the socket must be navigated with 58° of abduction and 32° of anteversion in the anatomical coordinate system (see Appendix).
In summary, we have shown that pelvic tilt is not neutral for most patients and that the pelvic tilt angle with the patient supine is a good approximation of the functional pelvic tilt angle with the patient standing. Computer navigation has reestablished the importance of the pelvic tilt angle as an objective measurement tool for cup position. We have shown that the criteria of Lewinnek et al. that reference the conventional coordinate system3 are applicable to navigation that references the anatomical coordinate system if pelvic tilt is determined and corrected for. We recognize that the concepts can be confusing, but we believe that we have shown that ignoring pelvic tilt can make navigation systems inaccurate; this is the rationale for tilt-adjusted navigation.
Our study was composed of a high proportion of women as well as patients with hip dysplasia (Tables I and II), which particularly highlighted the forward rotation and variability of pelvic tilt angle and the necessity of tilt adjustment of angles during navigation. Our nomograms were calculated with the assumption of a pure rotation of the pelvis either forward or backward. We then manually read target values from our graphs. We have assumed that rotations along other axes produced a negligible impact on recommended cup angles in the anatomical coordinate system. The next generation of software should therefore handle these possibilities automatically.
In the future, individual three-dimensional-impingement analysis that not only takes into account important parameters such as range of motion, neck angle, and neck anteversion13,17,18,27 but that also considers the problem of pelvic tilt in the supine, standing, and sitting positions should be developed as an additional tool of navigation systems.
Nomograms for tilt-adjusted abduction and anteversion for navigation in the anatomical coordinate system with the target values are available with the electronic versions of this article, on our web site at jbjs.org (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). 
Note: This study is part of the project BA 2229/1-1 and 2229/1-2 supported by the Deutsche Forschungsgemeinschaft (DFG).
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